Ametropia, an undesirable refractive condition of the eye, has three main subdivisions; myopia, hyperopia, and astigmatism. FIG. 1 is a schematic representation of an eye which shows parallel light rays 1 focusing on the retina 2 at F1. In myopia, by far the most common type of ametropia, the parallel light rays 1 which enter the eye as shown in FIG. 2 come to a focus F2 in front of the retina 2 as shown in FIG. 2. In hyperopia, the rays of light 1 come to a focus F3 behind the retina 2 as shown in FIG. 3. When the rays of light converge to not one, but several foci, it is referred to as astigmatism, in which condition the various foci may all lie before the retina; all lie behind the retina; or partly before and partly behind the retina.
Ametropia is usually corrected by glasses or contact lenses. However, these refractive disorders may also be corrected by surgery. Refractive eye surgery is defined as that surgery on the eye which acts to change the light-bending qualities of the eye. More common current refractive procedures include radial keratotomy, as described in U.S. Pat. Nos. 4,815,463 and 4,688,570 and also laser ablation of corneal stroma, described in U.S. Pat. No. 4,941,093. Various other surgical methods for the correction of refractive disorders have been tried including thermokeratoplasty for the treatment of hyperopia, epikeratoplasty to correct severe hyperopia, and keratomileusis which can steepen or flatten the central cornea. Keratomileusis was introduced by Barraquer of Columbia in 1961 and essentially involves grinding a corneal button into an appropriate shape to correct the refractive error and replacing the reshaped corneal button. Some of the more common keratorefractive procedures are discussed below, none of which have currently shown itself to have all the characteristics of an ideal keratorefractive procedure. The disadvantages of corneal refractive surgery include limited predictability, lack of reversibility, corneal destabilization, optical zone fibrosis, post-operative discomfort, and visual symptoms such as glare, halos, and starbursts.
The ideal keratorefractive procedure possesses the following characteristics: adjustability, predictability, good quality of vision (absence of post-operative glare and haloes), reversibility, stability over time, safety, and low cost. Any refractive procedure that attempts to achieve visual outcomes similar to that which can be provided by glasses must incorporate the ability to post-operatively adjust the refractive outcome since there will be variability in outcome from either the surgical procedure or the wound healing response. Photorefractive Keratectomy (PRK), a currently popular procedure, hopes to achieve adjustability with re-ablation. There is little doubt that PRK reduces the variability of the surgical procedure and thus improves predictability. However, the percentage of patients achieving an uncorrected visual acuity of 20/20 is still only 50-60%. "Enhancement" procedures are performed when the patient is undercorrected. But since the surgical variability is reduced and the variable wound healing responsible for the bulk of the variability of the refractive outcome, it becomes necessary to have an "enhancement" procedure that minimally causes a wound healing response. Unfortunately, the only "fine-tuning" procedure available with PRK is yet another PRK. The supposedly second "enhancement" PRK also involves removal of the corneal epithelium and ablation of the corneal stroma. In essence, the fine-tuning procedure is just as invasive as the ball-parking procedure and is no more predictable than the initial procedure. Rather, the enhancement procedure has the potential to invoke a greater wound-healing response since the corneal stroma has undergone previous insult. It can be seen that a refractive procedure that truly allows adjustability must minimally disturb the cornea and cause a minimal wound-healing response.
For years it has been thought that refractive surgery with intracorneal implants could be used in the correction of ametropia. Early techniques included lamellar removal or addition of natural corneal stromal tissue, as in keratomileusis and keratophakia. These required the use of a microkeratome to remove a portion of the cornea followed by lathing of either the patient's (keratomileusis) or donor's (keratophakia) removed cornea. The equipment is complex, the surgical techniques difficult, and most disappointingly, the results quite variable. The current trend in keratorefractive surgery has been toward techniques that are less traumatic to the cornea, that minimally stimulate the wound healing response, and behave in a more predictable fashion. The use of alloplastic intracorneal lenses to correct the refractive state of the eye, first proposed in 1949 by Jose Barraquer, have been plagued with problems of biocompatibility, permeability to nutrients and oxygen, corneal and lens hydration status, etc. Other problems with these lenses include surgical manipulation of the central visual axis with the concomitant possibility of interface scarring.
More recent efforts toward the correction of refractive errors have focused on minimizing the effects of the wound healing response by avoiding the central cornea. There have been multiple attempts to alter the central corneal curvature by surgically manipulating the peripheral cornea. These techniques are discussed because of their specific relevance to this invention.
In the Foreword to the textbook, Principles and Practice of Refractive Surgery, Jose Barraquer writes, "As a result of my initial publications, some authors decided to try different methods to modify the shape of the cornea. Punch stromectomy and temporal inclusion of a plastic disk by Krawawicz (1960), the use of a trephine by Pureskin (1967), soto impronta by Strampelli (1964), molding by Martinez and Katsin (1965), and corneal rings by Blawatkaia (1966) all were tried."
Zhivotosvskii, D. D., USSR Patent No. 3887846, describes an alloplastic, flat, geometrically regular, annular ring for intracorneal implantation of an inside diameter that does not exceed the diameter of the pupil. Refractive correction is accomplished primarily by making the radius of curvature of the surface of the ring larger than the radius of curvature of the surface of a recipient's cornea in order to achieve flattening of the central area of the cornea. Surgical procedures for inserting the ring are not described.
A. B. Reynolds (U.S. Pat. No. 4,452,235) describes and claims a keratorefractive technique involving a method and apparatus for changing the shape of the optical zone of the cornea to correct refractive error. His method comprises inserting one end of a split ring shaped dissecting member into the stroma of the cornea, moving the member in an arcuate path around the cornea, releasably attaching one end of a split ring shaped adjusting member to one end of the dissecting member, reversibly moving the dissecting member about the path, and thereby pulling the adjusting member about the circular path, made by the dissecting member, withdrawing the dissecting member, adjusting the ends of the split ring shaped adjusting member relative to one another to thereby adjust the ring diameter to change the diameter and shape of the cornea and fixedly attaching the ring's ends by gluing to maintain the desired topographical shape of the cornea.
A major advantage of this ring was that a very minimal wound healing effect was expected. A marked corneal wound healing response would decrease the long-term stability of any surgical refractive procedure. However, there are two distinct problem areas affecting the refractive outcome of surgical procedures treating ametropia:
1. The first problem is concerned with the ability to predetermine the shape and size of a implant that will lead to a certain refractive outcome. In RK or PRK, retrospective studies have been performed that led to the development of nomograms which predict that a certain depth cut or a certain ablation amount will result in a predictable amount of correction. In the case of the ring, eventually nomograms will be developed that can be used to predict a given refractive correction for a given thickness or size of the ring. However, these nomograms can never fully account for individual variability in the response to a given keratorefractive procedure. PA1 2. The refractive outcome also depends on the stability of the refractive correction achieved after surgery. To reiterate, the advantage of the ring would be the stability of the refractive outcome achieved because of a presumed minimal wound healing response. This decreases the variability of the long-term refractive outcome but still does not address the problems posed in the first problem area,--the inherent individual variability, in that while the outcome may be stable, it may very well be an inadequate refractive outcome that is stable.
Another unaddressed issue is that even with the implant, surgeons will aim for a slight under-correction of myopia because, in general, patients are more unhappy with an over-correction that results in hyperopia. Again, the refractive outcome may be more stable than in RK or PRK but it may be an insufficient refractive result that is stable.
Simon in U.S. Pat. No. 5,090,955 describes a surgical technique that allows for modification of the corneal curvature by inter-lamellar injection of a synthetic gel at the corneal periphery while sparing the optical zone. He does discuss removal of gel to decrease the volume displaced and thus adjust the final curvature of the central corneal region.
Siepser (U.S. Pat. No. 4,976,719) describes another ring-type device to either flatten or steepen the curvature of the cornea by using a retainer ring composed of a single surgical wire creating a ring of forces which are selectively adjustable to thereby permit selective change of the curvature of the cornea,--the adjustable means comprising a turnbuckle attached to the wire.
The mechanism by which corneal rings flatten the central corneal curvature is aptly described by J. Barraquer in the following quote. Since 1964, "It has been demonstrated that to correct myopia, thickness must be subtracted from the center of the cornea or increased in its periphery, and that to correct hyperopia, thickness must be added to the center of the cornea or subtracted from its periphery." Procedures involving subtraction were called `keratomileusis` and those involving addition received the name of `keratophakia`. Intrastromal corneal rings add bulk to the periphery and increasing the thickness of the ring results in a more pronounced effect on flattening of the anterior corneal curvature by "increasing (thickness) in its periphery".
The ideal keratorefractive procedure allows all the advantages of eyeglasses or contact lenses, namely, being able to correct a wide range of refractive errors, accuracy or predictability, allowing reversibility in the event that the refractive state of the eye changes and it becomes necessary to adjust the correction again, yielding minimal complications, and associated with technical simplicity, low cost, and being aesthetically acceptable to the patient. The goal of refractive surgeons should be to achieve 20/20 uncorrected visual acuity with long-term stability in greater than 95% of patients. None of the currently available refractive surgery procedures generate this degree of accuracy or stability.
Once again, an easy procedure to post-operatively fine-tune the refractive correction and corneal curvature which is often influenced by changes in corneal hydration status, wound healing responses, and other unknown factors, is not available. Each of the techniques described suffers from a limited degree of precision either due to a variable surgical procedure, a variable wound healing response, or variability in both the surgical procedure and wound healing. In this disclosure of the present invention, an easy method to adjust the refractive outcome after the corneal curvature has stabilized, a method that is minimally invasive, a method causing minimal stimulation of the wound healing processes, allowing repetitive adjustments as deemed necessary, and being almost completely reversible is described. It may make moot the pervasive issue of unpredictability and make obsolete the application of procedures which rely heavily upon nomograms to predict refractive outcome and are thus unable to adequately account for an individual's variable response to the procedure.